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A role for the small G protein rho and rho-kinase has been shown in smooth muscle contraction regarding Ca++ sensitivity. However, there are no data in the literature assessing how this system operates in human umbilical arteries (HUA). Therefore, we evaluated the effects of HA-1077 and Y-27632, two rho-kinase inhibitors, on agonist-(5-hydroxytryptamine [5-HT]) and depolarization-induced (KCl) contractions of HUA. HA-1077 and Y-27632 inhibited 5-HT-induced contractile responses at 10-4M concentration but not at 10(-5) M. HA-1077 at 10(-4) M also significantly attenuated contractions induced by 20 mM KCl. In addition, HUA precontracted with 5-HT relaxed concentration dependently in response to HA-1077 and Y-27632. When precontracted with KCl, HUA also relaxed dose-dependently in response to HA-1077, but the maximal relaxation was significantly smaller than the response obtained when precontracted with 5-HT. To determine possible involvement of rho-kinase on agonist-induced intracellular calcium-mediated contractions, tissues were precontracted with 5-HT in Ca++-free Krebs solution before cumulative addition of HA-1077 or Y-27632 (10(-7) to 10(-4) M). Both rho-kinase inhibitors relaxed HUA completely. Maximum relaxations of HUA to HA-1077 and Y-27632 were significantly larger than the responses seen in normal Krebs solution and were obtained with lower concentrations of the drugs considered to be more specific for rho-kinase inhibition. However, preincubation of HUA with HA-1077 or Y-27632 (10(-5) M for both) did not affect the 5-HT-induced contractions in this medium. Finally, immunoblot experiments revealed the expression of rho-kinase isoform rockII protein in HUA. These results indicate that rhoA/rho-kinase pathway can contribute to agonist-induced contractions of HUA. However, this effect appears to be limited to intracellular calcium-induced contractions and may be more important in sustaining contractions rather than the initial phase of force development.

This study was conducted to determine if mitogen-activated protein kinases (MAPKs) such as extracellular signal-regulated kinase (ERK1/2) contribute to endotoxin-induced vascular hyporeactivity via nitric oxide (NO) and/or prostacyclin (PGI₂) production in the rat isolated thoracic aorta. Incubation of endothelium-intact rings with endotoxin (100 µg/ml) for 4 h decreased the E_max value and increased the EC₅₀ value of norepinephrine. The endotoxin-induced increase in the EC₅₀ value of norepinephrine was decreased by phenylene-1,3-bis[ethane-2-isothiourea] dihydrobromide (1,3-PBIT), a selective inducible NO synthase inhibitor, and U0126, a selective inhibitor of ERK1/2 phosphorylation by MAPK kinase. The endotoxin-induced decrease in the E_max value of norepinephrine was reversed by 1,3-PBIT and further decreased by U0126. 1,3-PBIT and U0126 decreased the endotoxin-induced increase in the tissue nitrite and 6-keto-PGF_1α levels. These data suggest that events related to the activation of ERK1/2 contribute to the endotoxin-induced hyporeactivity by increasing NO and PGI₂ production.

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